This invention relates generally to voltage controlled oscillators (VCOs), and more particularly, to a VCO having cascaded emitter follower buffer stages.
VCOs typically have to operate in narrow frequency bands. For example, in some Industrial, Scientific and Medical (ISM) applications VCOs may have to operate in a 120 MHz frequency window. VCOs are typically phase locked to an input signal using a phase locked loop (PLL) that includes a feedback to control the output frequency of the VCO. The PLL maintains the VCO in the narrow frequency band.
VCOs are used in many different applications. For example, VCOs are used in transmitters to control the operating frequency of the transmitters. In high frequency applications, for example, when using a 24 GHz VCO, such as, used in automotive or other short range radars, isolation between the VCO and the other portions of the transmitter is an issue. When proper isolation is not provided, the frequency of the VCO is changed, often referred to as frequency pulling, as a result of varying load conditions. The varying load conditions may be caused, for example, by a change in environmental conditions, such as from varying reflections from a changing environment near the antenna, or by other circuits connected to the VCO changing the operating mode or state. For example, the turning on/off of the switches in a pulsed radar transmitter may vary the load impedance presented to the VCO. These changes in load-impedance (and hence frequency) can occur within nanoseconds at a rate that the PLL cannot correct. The operating frequency of the VCO will thus be affected and that may cause the VCO to operate outside of a desired or required frequency band, as well as having other affects on circuit performance, which may be less detectable.
It is known to provide isolation for a VCO in a transmitter application using a cascode output configuration or using buffer stages. For example, a plurality of buffer stages may be added in a serial arrangement to provide isolation of the VCO. These buffer stages provided in a serial arrangement increase the amount of current needed due to each stage requiring a bias current. As a result of the increased current, direct current (DC) consumption is increased, thereby increasing the amount of DC power needed. The increased current also results in an increase in temperature of the VCO, which in applications that present harsh thermal environments may result in unacceptable operation of the VCO (e.g., VCO operating outside of required frequency band). Thus, although isolation of VCOs may be provided using these known methods, the overall operation of the system may be adversely affected.